1. Field of the Invention
This invention relates to a spectrally sensitized silver halide photographic emulsion and more particularly, it is concerned with a silver halide photographic emulsion supersensitized by a novel combination of sensitizing dyes of two types.
2. Description of the Prior Art
It is well known that the sensitive wavelength range of a silver halide emulsion can be enlarged and spectrally sensitized by adding a sensitizing dye to the silver halide photographic emulsion. In some cases, only one sensitizing dye is used in order to sensitize the emulsion to a desired spectral sensitization wavelength, but two or more sensitizing dyes in combination are used often. When two or more sensitizing dyes are used in combination, the spectral sensitization achieved is between the case of using these sensitizing dyes individually or is often decreased. In some cases, however, the use of a specific combination of sensitizing dyes of different types results in a more marked increase of spectral sensitization than in the case of using the sensitizing dyes individually. This phenomenon is usually called "supersensitization".
When such a combination is used, the spectral sensitization wavelength is intermediate or a mere combination of the spectral sensitization wavelength obtained when the sensitizing dyes are used individually, but sometimes a shift of the spectral sensitization occurs to a wavelength which cannot be predicted from the spectral sensitization properties when the sensitizing dyes are used individually.
Thus, discovery of combinations of sensitizing dyes, which are capable of providing a higher spectral sensitization than in the case of using the sensitizing dyes individually and having a sensitization wavelength region suitable for the intended use of the photographic light-sensitive material, is very important in the spectral sensitization techniques of silver halide photographic emulsions.
In the combination of sensitizing dyes used for obtaining supersensitization, the sensitizing dyes must strictly be chosen since supersensitization is markedly affected even by an apparently small difference in chemical structure. That is to say, which combination of sensitizing dyes will be capable of providing supersensitization cannot readily be predicted from the chemical structures only.
The sensitizing dyes used in supersensitization of a silver halide photographic emulsion must not interact with other photographic additives and have stable photographic properties even after storage of the photographic lightsensitive material.
Another requirement for the sensitizing dyes used is that no residual coloration due to the sensitizing dyes must remain in the light-sensitive material after processings. In processing in a short time (from about 3 to about 90 seconds) such as rapid processing, in particular, a residual coloration must not be retained. Residual coloration is particularly disadvantageous in photographic papers, since a bright white color can not be obtained in black-and-white photography and reproduction of real colors is impossible in color photography.
It is well known as described in U.S. Pat. Nos. 3,752,670 and 3,480,434, German Patent Application (OLS) No. 2,303,204 and Japanese Patent Publication No. 30023/1971 to use sensitizing dyes in order to increase the sensitivity in the blue color region, but the photographic properties of the sensitizing dyes used in this sensitization of the blue color region are not sufficient. That is to say, the simple merocyanine dyes described in U.S. Pat. No. 3,480,434 and Japanese Patent Publication No. 30023/1971 have the disadvantage that the spectral sensitivity distribution of the silver halide emulsion is extended to a long wavelength side and the green region is also sensitized. This deteriorates the color reproduction in a multi-layer color photographic material. If a yellow filter layer is provided on the blue-sensitive layer so as to overcome this disadvantage, the sensitivity of the green region can be reduced to some extent, but, at the same time, the sensitivity of the blue region is also reduced, which is disadvantageous. Some of the monomethinecyanine dyes described in U.S. Pat. No. 3,752,670 and German Patent Application (OLS) No. 2,303,204 show a sharp J-band sensitization, but, have the disadvantage that the residual coloration is large. Therefore, an improved photographic emulsion is required in which the long wavelength side of spectral sensitization region in the blue region is sharply cut and the photographic element processed is scarcely dye-contaminated.